Dinitrotoluene (DNT) is an intermediate in the production of toluylene diisocyanate (TDI) which is an important precursor in the production of polyurethanes and is produced on a large industrial scale.
The production of dinitrotoluene by nitration of toluene with nitrating acid (mixture of nitric acid and sulfuric acid) has already been the subject of numerous publications and patent applications (Ullmanns Enzyklopedie der technischen Chemie, 4th edition, volume 17, page 391 ff, 1979, Verlag Chemie Weinheim/New York). As described for example in H. Hermann, J. Gebauer, P. Konieczny, “Industrial Nitration of Toluene to Dinitrotoluene” in ACS-Symposium, Series 623, 234-249, 1996, ed. L. F. Albright, R. V. C Carr, R. J. Schmitt, industrial production proceeds continuously in two stages in predominantly isothermal fashion with nitric acid in the presence of sulfuric acid as catalyst in such a way that                a) the reaction mixture obtained in the dinitration (nitration of mononitrotoluene—MNT—to DNT) is separated by phase separation and the thus obtained spent acid is reconcentrated with nitric acid and then mixed with toluene and supplied to the mononitration (nitration of toluene to MNT) and        b) once reaction has been effected the reaction mixture from the mononitration is resolved in a separation stage into an organic phase comprising the mononitrotoluene and an aqueous phase comprising predominantly the sulfuric acid (spent acid) and        c) the mononitrotoluene-comprising organic phase obtained in b) is supplied to the dinitration and the mononitrotoluene is reacted there with nitric acid in the presence of sulfuric acid to afford dinitrotoluene.        
To achieve commercial specifications the thus obtained crude DNT is typically processed in downstream stages, predominantly washes, and thus largely freed of dissolved sulfuric and nitric acid contents and also of secondary components formed in the reaction stages, for example mono-, di- and trinitrocresols (referred to hereinbelow simply as nitrocresols), picric acid and nitrobenzoic acids. Typical commercial DNT products have DNT contents >98.5 wt %, less than 0.1 wt % of mononitrotoluene, less than 0.1 wt % of trinitrotoluene and less than 0.1 wt % of other secondary components based on the weight of the DNT product mixtures with DNT yields of >98% and toluene conversions of >99.9%. Also significant is the weight ratio of the sum of the 2,4- and 2,6-DNT isomers to the sum of the 2,3-, 3,4-, 2,5- and 3,5-DNT isomers. According to commercial specifications the content of the sum of the 2,4- and 2,6-DNT isomers in the crude DNT is >95 wt % based on the weight of the crude DNT. It is preferable when the content of 2,4-DNT is 79.0-81.0 wt % based on the sum of the weights of 2,4-DNT and 2,6-DNT. Accordingly, the content of 2,6-DNT is 19.0-21.0 wt % based on the sum of the weights of 2,4-DNT and 2,6-DNT.
In addition to the crude DNT, in the resolution of the reaction mixture obtained in the mononitration, the process affords spent acid which leaves the system as a second mass flow. The spent acid typically has a sulfuric acid content of 70-80 wt % and typically comprises >0.1, preferably >0.2 to 3.0 wt % of unconverted nitric acid, nitrose from oxidation processes occurring in secondary reactions, >0.2 wt % of MNT not separated in the phase separation and typically water in a concentration range of >16 to <30 wt % (comprising water introduced with the sulfuric acid freshly employed in the process, water present in the nitric acid and water formed during the nitrations of the toluene and of the mononitrotoluene) in each case based on the weight of the spent acid.
EP 1 496 043 A1 describes a process for the workup of aqueous wastewaters generated during the nitration of toluene to dinitrotoluene with nitrating acid, wherein the acidic and alkaline wastewaters from the dinitrotoluene wash and the aqueous distillate from the sulfuric acid concentration are combined so that a pH of below 5 (measured at 70° C.) is established. The aqueous phase and organic phase formed are then separated by phase separation. The organic components present in the aqueous phase are extracted with toluene and the toluene phase enriched with organic components is then supplied to the nitration of toluene. The extraction described is a step distinct from the crude dinitrotoluene wash. The application further discloses that the aqueous phase from the extraction may be supplied to a steam stripping. The obtained water vapor-toluene mixture is condensed and the toluene present in the condensate may be recycled into the nitration after phase separation. The application does not disclose subjecting offgas streams from the gas spaces of the apparatuses employed in the nitration reaction, in the wash and/or in the DNT-water phase separation to a condensation to remove toluene.
The nitrogen oxides (NOx) formed in the nitration may be treated with aqueous alkali metal hydroxide solution and washed out as sodium nitrate and nitrite as described in U.S. Pat. No. 5,313,009. In addition, carbon dioxide formed in the nitration process is bound as sodium carbonate.
U.S. Pat. No. 5,963,878 discloses a process wherein NOx gases are obtained from strategic regions of the nitration system, brought into contact with air and water, for example in a unit comprising a packed bed, at relatively high temperatures and under pressure, wherein the NOx gases are absorbed by the water to form weak nitric acid. The weak nitric acid is recycled into the reaction process. Carbon dioxide is not absorbed in a NOx gas scrubbing tower when the gas scrubbing tower is operated in an acidic mode. Clean NOx-free vent gas is discharged as flue gas from the unit comprising a packed bed.
Common to all of the processes is that a further treatment of the offgases from nitration plants is not provided for.
EP1880989A1 describes that numerous past studies sought to improve the quality of the crude DNT and thus to increase the yield based on toluene and nitric acid. For all processes for producing DNT by nitration of toluene with nitric acid it is a prerequisite for economic running of the process that the spent acids generated during the process may be reprocessed in such a way that they may be reemployed as reaction medium in the reaction process (as described in EP 155 586 A and U.S. Pat. No. 5,275,701 A for example).
However, other significant factors affecting the choice of a DNT nitration process also include its inherent safety, the robustness with which it can be operated, the selectivity with and extent to which the toluene can be converted to dinitrotoluene, and the specific use of nitric acid necessary for the conversion of the toluene to dinitrotoluene. It is thanks to these developments that the current DNT processes have reached a level of maturity which allows all of them to produce DNT in high yields with a low content of byproducts.
However, it was found during operation of a nitration plant for producing dinitrotoluene from toluene that the isolated molar amount of dinitrotoluene end product and of the byproducts inherently associated with the process such as cresols and their degradation products and also mononitrotoluene and trinitrotoluene was lower than the corresponding amount of toluene employed. The outlet for this loss is evidently the production plant offgas which is undesirable from an ecological standpoint. The loss is also associated with economic disadvantages.
There was therefore a need to improve existing nitration technology to reduce environmental contamination with organics via the offgas. It was furthermore sought ideally to effect the reduction in the environmental contamination such that it is associated with economic advantages. It was sought in particular to send toluene entrained in the offgas for economic recovery.